1. Field of the Invention
This invention relates to the selective dissolution of bismuth from oxidized particulate materials using an acidic chloride solution.
2. The Prior Art
A common source of particulate matter having a recoverable quantity of metallic values is flue dust which is the extremely finely divided particulate matter that is recovered by filtration and electrostatic precipitation from the exhaust stacks of smelters, furnaces or the like. Generally, the primary metal constituent of the smelting operation is also reflected as the primary metal constituent of the flue dust. Accordingly, the recovery and extraction of metal values from flue dust represents an important economic consideration in the extractive metals industry.
For example, flue dust from a typical smelter is captured by conventional filter systems and may be processed to recover the metal values. Processing may involve a selective sulfation roast at about 450.degree.C. In the case of flue dust from a zinc smelter, the principal products from the roasting process are zinc oxide, cadmium sulfate, and lead sulfate. Lesser amounts of other metallic compounds are also present. Typically, water and sulfuric acid soluble metallic compounds are dissolved and their values recovered by appropriate solution purification techniques.
A water leach of the roasted flue dust results in a dissolution of approximately 80% of the water-soluble compounds. The undissolved residue from the water leach is next subjected to dilute sulfuric acid leach to dissolve most of the acid-soluble compounds. The undissolved residue from this acid leach is referred to in the industry as lead cake residue, an example of the type of material with which this invention is concerned.
Residues such as lead cake residue contain a significant amount of silver (sometimes in excess of 100 ounces per ton) and the residue may, therefore, represent a valuable product for its silver content alone. For example, a typical analysis of lead cake residue from a zinc smelter revealed the following metallic content: lead 44.22%, zinc 2.72%, cadmium 0.93%, copper 0.95%, antimony 0.019%, arsenic 0.065%, indium 0.015%, bismuth 0.051% and silver at 109 ounces per ton. Nonmetallic elements in chemical combination with the metals complete the remainder of the percentage analysis.
The full economic potential of such residues frequently cannot be realized due to the penalties imposed for the bismuth content or due to undeveloped technology for the recovery of major components. Flue dust from a zinc smelter is one example in which the bismuth content is generally considered to be an impurity and its removal is, therefore, required due to product specifications. Specifically, lead smelters frequently will not accept secondary raw materials such as flue dusts containing more than about 0.02% bismuth. The reason for this penalty is that bismuth is known to interfere with the recovery of silver values from lead cake residue and also alter the physical properties of the metallic lead particularly with respect to its ductility. Consequently, such residues must be sold elsewhere at less than the premium price it would otherwise demand if the bismuth were not present or were present in amounts less than about 0.02%.
Until the present invention, the removal of bismuth from residues has generally not been commercially practiced successfully because of deficient technology and economic factors encountered. For example, a bismuth content of 0.051%, as in the foregoing zinc smelter lead cake residue, does not represent a realistic level for recovering bismuth as a metallic value, although the removal of bismuth to a level less than 0.02% would contribute significantly to enhancing the value of the lead cake residue.
On the other hand, other flue dust residues may contain a significant bismuth concentration whereby bismuth may be considered one of the primary metallic values. For example, flue dust from a copper smelting operation has been analyzed and found to contain approximately 3% bismuth. This residue has been stockpiled at one copper smelter alone in an accumulation that has amounted to approximately 10,000 tons. In this instance, the 3% bismuth level represents a significant metallic value of bismuth, the recovery of which should be given economic considerations provided the necessary technological processing can be developed such as is described in this invention.
In view of the foregoing, what is needed is a process to selectively separate and remove bismuth from oxidized particulate materials and waste products such as flue dust residues and roasted concentrates. The bismuth removal process should not substantially affect the silver content, but rather maintain the stability of lead and silver in the solid phase. Preferably, the process should include the selective dissolution of bismuth only, without substantially affecting the solubility of either the lead or silver content of the residue. Such an invention is disclosed herein.